In recent years, to reduce the amount of CO2 emission in view of global environmental protection, there has been a strong demand for improved fuel efficiency in automobiles. Thus, there is an active movement toward reduction in vehicle body weight by improving the strength of vehicle body members while simultaneously reducing the sheet thickness. To this end, high strength steel sheet having TS 590 MPa or more are widely utilized to produce a cold rolled steel sheet and hot-dip galvanized steel sheet, which are formed into vehicle body members by press working. Moreover, to ensure satisfactory collision safety characteristics required for automobiles, enhanced absorption of the collision energy is mandatory. To improve the collision-energy absorbing property, an effective measure is to increase the yield ratio. The higher the yield ratio, the more effectively the collision energy be absorbed even with a small volume of deformation.
On the other hand, a high strength steel sheet reduced in thickness sees significant impairment in shape fixability. For this reason, it is a widespread practice to perform press forming by predicting change in shape of pressed parts separated from the mold to design the press mold in expectation of the change in shape. If the tensile strength of the steel sheet is changed significantly, the actual change in shape largely deviates from the expected change based on the assumption that the tensile strength would remain unchanged, which leads to shape defects, making indispensable the procedure of subjecting the pressed parts one by one to sheet metal processing for shape correction, with the result that mass-production efficiency is significantly deteriorated. In view of this, there has been a demand for cold rolled steel sheets and hot-dip galvanized steel sheets with minimized difference in strength, that is, having excellent material homogeneity.
In this regard, as a mechanism to reinforce the steel sheet to have a tensile strength of 590 MPa or more, it is known to harden ferrite as the matrix phase or utilize a hard phase such as martensite. Among those described above, precipitation-strengthened, high strength steel sheet obtained by adding carbide-forming elements such as Nb makes it possible to readily improve the yield ratio and reduce the amount of alloying elements necessary to realize a predetermined strength, thereby lowering the production cost.
For instance, JP 2008-174776 A discloses a high strength thin steel sheet having a tensile strength of at least 590 MPa excellent in stretch flange formability and impact energy absorption property, the steel sheet being strengthened by precipitation through the addition of Nb and Ti. JP 2008-156680 A discloses a high strength cold rolled steel sheet strengthened by precipitation through addition of Nb and Ti and has a steel sheet microstructure containing a recrystallized ferrite, non-recrystallized ferrite, and pearlite.
Further, a high strength cold rolled steel sheet is significantly affected by the steel sheet structure and precipitated amounts in the hot-rolled steel sheet and, thus, it would be effective to realize a higher strength in the hot-rolled steel sheet. In relation to the hot-rolled steel sheet, JP 3767132 B2 discloses a method of producing a hot-rolled steel sheet having excellent ductility and material homogeneity by controlling the Nb and Ti contents. On the other hand, JP 2000-212687 A discloses a hot-rolled steel sheet having improved material homogeneity and hole expansion formability by controlling the Ti content.
However, according to JP '776, the Al content in the steel sheet is less than 0.010%. Thus, deoxidation of steel and fixation of N as precipitates are insufficiently performed, making it difficult to mass-produce sound steel. In addition, the steel contains oxygen (O) and has oxides dispersed therein, which leads to a problem in that the steel considerably fluctuates in material quality. Further, according to JP '680, a non-recrystallized ferrite is uniformly dispersed to thereby suppress deterioration in ductility, but no consideration is given to material homogeneity. In addition, according to JP '776 and JP '680, microstructural control after cold rolling is performed to thereby improve ductility and reduce variation in ductility in the width direction, but no consideration is given to microstructural control during hot rolling.
Further, JP '132 and JP '687 disclose methods of producing a hot-rolled steel sheet having high ductility or hole expansion formability, but the hot-rolled steel sheet thus obtained is not considered as a hot-rolled material to produce a cold rolled steel sheet and as a hot-rolled material to produce a hot-dip galvanized steel sheet. Thus, it would be highly desirable to develop a hot-rolled steel sheet excellent in material homogeneity after annealing, and which can be suitably utilized as a material to produce a cold rolled steel sheet and hot-dip galvanized steel sheet.
Therefore, it could be helpful to provide a hot-rolled steel sheet which can be suitably utilized to produce a cold rolled steel sheet or hot-dip galvanized steel sheet each having a tensile strength of 590 MPa or more, excellent in material homogeneity and capable of giving excellent cold rolling property.